Concanavalin A Delivers a Photoactive Protein to the Bacterial Wall

Modular supramolecular complexes, where different proteins are assembled to gather targeting capability and photofunctional properties within the same structures, are of special interest for bacterial photodynamic inactivation, given their inherent biocompatibility and flexibility. We have recently proposed one such structure, exploiting the tetrameric bacterial protein streptavidin as the main building block, to target S. aureus protein A. To expand the palette of targets, we have linked biotinylated Concanavalin A, a sugar-binding protein, to a methylene blue-labelled streptavidin. By applying a combination of spectroscopy and microscopy, we demonstrate the binding of Concanavalin A to the walls of Gram-positive S. aureus and Gram-negative E. coli. Photoinactivation is observed for both bacterial strains in the low micromolar range, although the moderate affinity for the molecular targets and the low singlet oxygen yields limit the overall efficiency. Finally, we apply a maximum entropy method to the analysis of autocorrelation traces, which proves particularly useful when interpreting signals measured for diffusing systems heterogeneous in size, such as fluorescent species bound to bacteria.

In Situ Bioconjugation of a Maleimide-Functionalized Ruthenium-Based Photosensitizer to Albumin for Photodynamic Therapy

Maleimide-containing prodrugs can quickly and selectively react with circulating serum albumin following their injection in the bloodstream. The drug-albumin complex then benefits from longer blood circulation times and better tumor accumulation. Herein, we have applied this strategy to a previously reported highly phototoxic Ru polypyridyl complex-based photosensitizer to increase its accumulation at the tumor, reduce off-target cytotoxicity, and therefore improve its pharmacological profile. Specifically, two complexes were synthesized bearing a maleimide group: one complex with the maleimide directly incorporated into the bipyridyl ligand, and the other has a hydrophilic linker between the ligand and the maleimide group. Their interaction with albumin was studied in-depth, revealing their ability to efficiently bind both covalently and noncovalently to the plasma protein. A crucial finding is that the maleimide-functionalized complexes exhibited significantly lower cytotoxicity in noncancerous cells under dark conditions compared to the nonfunctionalized complex, which is a highly desirable property for a photosensitizer. The binding to albumin also led to a decrease in the phototoxicity of the Ru bioconjugates in comparison to the nonfunctionalized complex, probably due to a decreased cellular uptake. Unfortunately, this decrease in phototoxicity was not compensated by a dramatic increase in tumor accumulation, as was demonstrated in a tumor-bearing mouse model using inductively coupled plasma mass spectrometry (ICP-MS) studies. Consequently, this study provides valuable insight into the future design of in situ albumin-binding complexes for photodynamic therapy in order to maximize their effectiveness and realize their full potential.

The Interaction of Hypericin with SARS-CoV-2 Reveals a Multimodal Antiviral Activity

Hypericin is a photosensitizing drug that is active against membrane-enveloped viruses and therefore constitutes a promising candidate for the treatment of SARS-CoV-2 infections. The antiviral efficacy of hypericin is largely determined by its affinity toward viral components and by the number of active molecules loaded on single viruses. Here we use an experimental approach to follow the interaction of hypericin with SARS-CoV-2, and we evaluate its antiviral efficacy, both in the dark and upon photoactivation. Binding to viral particles is directly visualized with fluorescence microscopy, and a strong affinity for the viral particles, most likely for the viral envelope, is measured spectroscopically. The loading of a maximum of approximately 30 molecules per viral particle is estimated, despite with marked heterogeneity among particles. Because of this interaction, nanomolar concentrations of photoactivated hypericin substantially reduce virus infectivity on Vero E6 cells, but a partial effect is also observed in dark conditions, suggesting multiple mechanisms of action for this drug.

A photosensitizing fusion protein with targeting capabilities

The photodynamic treatment for antimicrobial applications or anticancer therapy relies on reactive oxygen species generated by photosensitizing molecules after absorption of visible or near-infrared light. If the photosensitizing molecule is in close vicinity of the microorganism or the malignant cell, a photocytotoxic action is exerted. Therefore, the effectiveness of photosensitizing compounds strongly depends on their capability to target microbial or cancer-specific proteins. In this study, we report on the preparation and preliminary characterization of human recombinant myoglobin fused to the vasoactive intestinal peptide to target vasoactive intestinal peptide receptor (VPAC) receptors. Fe-protoporphyrin IX was replaced by the photosensitizing compound Zn-protoporphyrin IX. Taking advantage of the fluorescence emission by Zn-protoporphyrin IX, we show that the construct can bind prostate cancer cells where the VPAC receptors are expressed.

Versatile Supramolecular Complex for Targeted Antimicrobial Photodynamic Inactivation

We report the development of a supramolecular structure endowed with photosensitizing properties and targeting capability for antimicrobial photodynamic inactivation. Our synthetic strategy uses the tetrameric bacterial protein streptavidin, labeled with the photosensitizer eosin, as the main building block. Biotinylated immunoglobulin G (IgG) from human serum, known to associate with Staphylococcus aureus protein A, was bound to the complex streptavidin-eosin. Fluorescence correlation spectroscopy and fluorescence microscopy demonstrate binding of the complex to S. aureus. Efficient photoinactivation is observed for S. aureus suspensions treated with IgG-streptavidin-eosin at concentrations higher than 0.5 μM and exposed to green light. The proposed strategy offers a flexible platform for targeting a variety of molecules and microbial species.

Photodynamic treatment of pathogens

The current viral pandemic has highlighted the compelling need for effective and versatile treatments, that can be quickly tuned to tackle new threats, and are robust against mutations. Development of such treatments is made even more urgent in view of the decreasing effectiveness of current antibiotics, that makes microbial infections the next emerging global threat. Photodynamic effect is one such method. It relies on physical processes proceeding from excited states of particular organic molecules, called photosensitizers, generated upon absorption of visible or near infrared light. The excited states of these molecules, tailored to undergo efficient intersystem crossing, interact with molecular oxygen and generate short lived reactive oxygen species (ROS), mostly singlet oxygen. These species are highly cytotoxic through non-specific oxidation reactions and constitute the basis of the treatment. In spite of the apparent simplicity of the principle, the method still has to face important challenges. For instance, the short lifetime of ROS means that the photosensitizer must reach the target within a few tens nanometers, which requires proper molecular engineering at the nanoscale level. Photoactive nanostructures thus engineered should ideally comprise a functionality that turns the system into a theranostic means, for instance, through introduction of fluorophores suitable for nanoscopy. We discuss the principles of the method and the current molecular strategies that have been and still are being explored in antimicrobial and antiviral photodynamic treatment.

Natural Photosensitizers in Antimicrobial Photodynamic Therapy

Health problems and reduced treatment effectiveness due to antimicrobial resistance have become important global problems and are important factors that negatively affect life expectancy. Antimicrobial photodynamic therapy (APDT) is constantly evolving and can minimize this antimicrobial resistance problem. Reactive oxygen species produced when nontoxic photosensitizers are exposed to light are the main functional components of APDT responsible for microbial destruction; therefore, APDT has a broad spectrum of target pathogens, such as bacteria, fungi, and viruses. Various photosensitizers, including natural extracts, compounds, and their synthetic derivatives, are being investigated. The main limitations, such as weak antimicrobial activity against Gram-negative bacteria, solubility, specificity, and cost, encourage the exploration of new photosensitizer candidates. Many additional methods, such as cell surface engineering, cotreatment with membrane-damaging agents, nanotechnology, computational simulation, and sonodynamic therapy, are also being investigated to develop novel APDT methods with improved properties. In this review, we summarize APDT research, focusing on natural photosensitizers used in in vitro and in vivo experimental models. In addition, we describe the limitations observed for natural photosensitizers and the methods developed to counter those limitations with emerging technologies.

Development of a light activatable lignin nanosphere based spray coating for bioimaging and antimicrobial photodynamic therapy

Many coating materials are commercially available to combat microbial infections. However, these coatings are difficult to synthesize, and are mostly composed of toxic chemicals. Lignin is an under-explored natural biopolymer with multifaceted potential. Lignin, with adhesive, UV resistant, and antimicrobial properties, is a suitable candidate to develop coating materials. Here we report a smart method to fabricate a sustainable nanospray coating from lignin which does not require any toxic chemicals or additives during synthesis. Initially, we have developed stable lignin nanospheres in a single step in aqueous medium, which were later utilized as a lignin nanospray (LNSR). The LNSR was characterized by dynamic light scattering, scanning electron microscopy, FTIR and other analytical techniques. This LNSR showed remarkable UV blocking, antioxidant and light-activated antimicrobial properties. Interestingly, for the first time, the LNSR demonstrated photoluminescence, making it useful for bioimaging. Moreover, singlet oxygen generation potential was observed in the LNSR, which could render it useful in phototheranostic applications (i.e. light assisted imaging and photodynamic therapy). Further, the LNSR was directly utilized to fabricate a sustainable coating. The nanospray coating exhibited maximum light-induced cell killing when applied to common microbes as detected by live-dead cell imaging. Taken together, the lignin nanospray coating developed via a direct pathway holds great promise to disinfect microbes in the presence of light.

Viral Nanoparticle System: An Effective Platform for Photodynamic Therapy

Photodynamic therapy (PDT) is a promising therapy due to its efficiency and accuracy. The photosensitizer is delivered to the target lesion and locally activated. Viral nanoparticles (VNPs) have been explored as delivery vehicles for PDT in recent years because of their favorable properties, including simple manufacture and good safety profile. They have great potential as drug delivery carriers in medicine. Here, we review the development of PDT photosensitizers and discuss applications of VNP-mediated photodynamic therapies and the performance of VNPs in the treatment of tumor cells and antimicrobial therapy. Furthermore, future perspectives are discussed for further developing novel viral nanocarriers or improving existing viral vectors.

Biophysical Characterization and Anticancer Activities of Photosensitive Phytoanthraquinones Represented by Hypericin and Its Model Compounds

Photosensitive compounds found in herbs have been reported in recent years as having a variety of interesting medicinal and biological activities. In this review, we focus on photosensitizers such as hypericin and its model compounds emodin, quinizarin, and danthron, which have antiviral, antifungal, antineoplastic, and antitumor effects. They can be utilized as potential agents in photodynamic therapy, especially in photodynamic therapy (PDT) for cancer. We aimed to give a comprehensive summary of the physical and chemical properties of these interesting molecules, emphasizing their mechanism of action in relation to their different interactions with biomacromolecules, specifically with DNA.

Design of Photosensitizing Agents for Targeted Antimicrobial Photodynamic Therapy

Photodynamic inactivation of microorganisms has gained substantial attention due to its unique mode of action, in which pathogens are unable to generate resistance, and due to the fact that it can be applied in a minimally invasive manner. In photodynamic therapy (PDT), a non-toxic photosensitizer (PS) is activated by a specific wavelength of light and generates highly cytotoxic reactive oxygen species (ROS) such as superoxide (O2-, type-I mechanism) or singlet oxygen (1O2*, type-II mechanism). Although it offers many advantages over conventional treatment methods, ROS-mediated microbial killing is often faced with the issues of accessibility, poor selectivity and off-target damage. Thus, several strategies have been employed to develop target-specific antimicrobial PDT (aPDT). This includes conjugation of known PS building-blocks to either non-specific cationic moieties or target-specific antibiotics and antimicrobial peptides, or combining them with targeting nanomaterials. In this review, we summarise these general strategies and related challenges, and highlight recent developments in targeted aPDT.